Composed of two symmetrical arranged conductors, the communication loop is available in high-frequency and low-frequency versions. The former supports transmission frequencies up to 800 kHz, enabling the opening of 180 telephone lines within a single loop; the latter typically has a higher transmission frequency below 252 kHz, which corresponds to 60 telephone lines per loop. The electromagnetic field of the symmetrical communication cable is in an open state, leading to significant attenuation and loss at high frequencies, as well as considerable mutual interference between loops and external interference, making it difficult to increase transmission frequency and capacity.

Long-haul symmetrical communication cables are composed of four-wire groups with varying numbers and insulation structures. The commonly used form of the four-wire group is the star绞, with some also using the complex pair绞 structure. Insulation options include paper tape, paper-rope (paper tape and paper rope), polyethylene rope-tape, polystyrene rope-tape, and foam polyethylene. High-frequency long-haul symmetrical communication cables transmit at high frequencies, thus requiring higher structural performance. Generally, a star绞 four-wire group with rope-tape insulation is used. Common insulation materials are polystyrene and polyethylene. Paper tape and paper rope insulation is typically used for low-frequency symmetrical communication cables below 252 kHz.

Telephone cables are symmetrical communication cables used for telephone connections within urban, suburban, and industrial and mining enterprises over smaller areas, commonly referred to as urban telephone cables. Their main feature is the high number of conductors (up to thousands, typically hundreds or dozens). Due to their lower frequency of use and shorter communication distances, the wire diameter is finer than that of long-distance communication cables, usually 0.5 millimeters. The wire group structures of telephone cables include paired twisting, star twisting, and composite paired twisting. They can be categorized according to their core insulation and sheath materials into paper-insulated lead-sheathed cables, polyethylene-insulated combined sheath cables, grease-filled waterproof cables, and all-plastic cables. Two insulated conductor cores in the telephone cable are twisted together at a certain pitch to form an insulated wire pair. The colors of the two insulated conductor cores in the pair are different (usually red and white) to facilitate identification during wiring. The cable core structure of telephone cables is generally divided into concentric and unitary types (see Figure 3). The twisting pitch of adjacent pairs in the same layer should be different to minimize mutual interference during calls. A marking pair (blue and white) is set in each layer to facilitate identification during wiring. In cables with 80 pairs or more, reserve pairs are often provided to replace不合格 pairs. Unitary cables are composed of 50 or 100 pairs and their corresponding reserve pairs to form a basic unit, which is then twisted together to form the cable core. After drying, the sheath is extruded to produce the cable (see illustration).

The overhead telephone cables require self-supporting design, with steel wires bearing the entire weight of the cable. Since the 1960s, waterproof cables have been developed, filled with grease within the plastic cables to prevent moisture and humidity.

Since the 1970s, to enhance the moisture resistance and stability of communication cables, a comprehensive cable sheath has been widely used in urban telephone cables. There are three basic types: 1. Alpeth: The cable core is extruded with a layer of polyethylene sheath, then wrapped longitudinally with a 0.2mm corrugated aluminum strip, filled with a polyisobutylene insulation composite, and coated with a thermoplastic adhesive layer externally. The outer layer is extruded with a polyethylene jacket. 2. Stalpeth: A layer of corrugated steel strip is wrapped around the corrugated aluminum strip, covered with a thermoplastic material, and then extruded with a polyethylene jacket. 3. Lepeth: The cable core is covered with a polyethylene sheath, coated with a sticky thermoplastic material externally, and then extruded with a lead tube.